A. Field of the Invention
The present invention relates to a process for devolatilizing an acrylic resin in the presence of a hindered amine light stabilizer having at least one aminoether group. The present invention also relates to a process for preparing a powder coating composition which includes a step of devolatilizing an acrylic resin and a composition capable of being formed into a powder coating composition.
B. Description of the Related Art
In one typical preparation of acrylic resins, copolymerizable compounds, usually monomers, are mixed in an appropriate solvent, such as xylene, along with an initiator and reacted to form the acrylic polymer. The solvent and residual monomers are then separated from the acrylic resin at an elevated temperature and reduced pressure thereby leaving the acrylic resin by a technique referred to as devolatilization.
Although acrylic resins have a number of utilities, one valuable utility is in the field of coatings, particularly powdered coatings, wherein the acrylic resin is melt blended with other coating components and additives, such as light stabilizers, which improve the characteristics of the coating. The blend is permitted to cool and is then formed into a powder that can be applied to various surfaces such as automotive parts and bodies.
One of the problems encountered with the devolatilization procedure is that the conditions under which the solvent and residual monomers are separated from the acrylic resin causes the acrylic polymer chain to degrade thereby forming monomers and low molecular weight oligomers. This phenomenon in turn causes a lower yield of the desired resin and batch to batch variations in the properties of the resin. Furthermore, the presence of monomers and oligomers function as plasticizing agents that can affect the processing characteristics of the resin and can lead to deficiencies, such as microporosity and yellowing, in coatings made therefrom.
Illustrative of the art relating to acrylic resins that are prepared and used in powder coatings is U.S. Pat. No. 5,663,240 which describes the preparation of a powder coating of an epoxy-functional acrylic copolymer and carboxylic acid. As set forth in the patent, an illustrative epoxy-functional acrylic copolymer (Polymer Example A) is prepared in a xylene solvent and subjected to devolatilization by vacuum distillation over a four hour period during which time the temperature of the batch increased to 356xc2x0 F. (180xc2x0 C.). In order to form the powder coating composition, the copolymer is blended with other components, including flow control additive, air release agent and ultraviolet light stabilizer, in a Henschel Blade Blender, extruded at an elevated temperature, a catalyst is added, the mixture is then further blended, extruded again, hammer milled and classified.
Additional powder coating compositions that are blended with various additives including light stabilizers are described in U.S. Pat. Nos. 5,212,245, 5,523,349 and 5,648,117 (divisional of the ""349 patent). A low gloss powder coating composition which also includes various additives including light stabilizers is set forth in U.S. Pat. No. 5,744,522 which also includes a background discussion of various other powder coating documents. A further patent relating to a powder paint composition is U.S. Pat. No. 6,008,301, which includes a discussion of documents relating to powder coating technology in the background section of the patent.
When coating articles such as automobile bodies and parts, it is often in order to use multiple coating layers with the top layer being referred to as a clear coat. Polymeric compositions are also used for the top layer and such compositions can include a variety of additives including light stabilizers. Illustrative patents which disclose polymeric compositions for a top coating layer are U.S. Pat. Nos. 5,547,757, 5,609,960 and 5,891,958.
In one aspect, the present invention provides a process for separating a solvent and low molecular weight material from an acrylic resin. The method involves adding a hindered amine light stabilizer having at least one aminoether group to a mixture of the organic solvent, the acrylic resin and the low molecular weight material and subjecting the resulting mixture to an elevated temperature for a time sufficient to remove the organic solvent and the low molecular weight material from the acrylic resin by devolatilization.
In a further aspect, the present invention provides a process for forming a powder coating composition. The method involves adding a hindered amine light stabilizer having at least one aminoether group to a mixture comprised of an organic solvent, low molecular weight material and an acrylic resin and subjecting the resulting mixture to an elevated temperature for a time sufficient to remove the organic solvent and low molecular weight material from the acrylic resin by devolatilization, blending the acrylic resin with components so as to form a composition suitable for coating a surface and forming the composition into a powder coating composition.
In a still further aspect, the present invention provides a composition capable of being formed into a powder coating composition which consists essentially of an acrylic resin, a hindered amine light stabilizer having at least one aminoether group or residue thereof and less than 4% by weight of low molecular weight material having a molecular weight less than 600.